Electron beam exposure method

ABSTRACT

An electron beam exposure method includes the steps of: preparing an exposure mask having a plurality of opening patterns formed by dividing a drawing object pattern into exposable regions; and drawing the drawing object pattern by performing exposure with an electron beam passing through the opening patterns of the exposure mask. Each end portion serving as a joint in each opening pattern of the exposure mask is provided with a joining portion tapered in a width of the opening pattern. The exposure is performed in such a way that portions drawn through adjacent joining portions overlap each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. application Ser. No.14/056,651 filed Oct. 17, 2013 which is based upon and claims thebenefit of priority of the prior Japanese Patent Application No.2012-235507, filed on Oct. 25, 2012, the entire contents of which areincorporated herein by reference.

FIELD

The embodiments discussed herein are related to an electron beamexposure method.

BACKGROUND

For a semiconductor device such as an LSI, a technique called siliconphotonics to establish connection to the semiconductor device throughoptical waveguides is currently under development to solve a signaldelay attributed to metal wiring.

The optical waveguide employs silicon which is translucent to light in anear infrared region, and has a pattern width of several hundreds ofnanometers. If this fine optical waveguide has roughness in the size ofseveral nanometers at an edge portion of a pattern, such roughness maycause a large loss and block transmission of optical signals.

Further, when an optical resonator is formed by using the opticalwaveguide, a wavelength of resonating light changes significantly evenwith slight variation in the width of a gap portion between patterns.

For these reasons, high-accuracy processing techniques are required tomanufacture optical waveguides and optical elements suitable forpractical use.

To this end, one conceivable method is to form a pattern in an opticalelement by using an electron beam exposure apparatus.

However, the line with of the optical waveguide is as large as severalhundreds of nanometers. Accordingly, if an electron beam spot of theelectron beam exposure apparatus is narrowed to secure requiredaccuracy, a large number of times of irradiation are necessary and ittakes long time to draw the pattern.

The above noted conventional technologies are described, for example, inJapanese Laid-open Patent Publication Nos. H08-195339 and 2011-49556.

SUMMARY

An object of the embodiments discussed herein is to provide an electronbeam exposure method capable of accurately and quickly drawing apattern.

According to an aspect to be disclosed below, there is provided anelectron beam exposure method which includes the steps of: preparing anexposure mask having a plurality of opening patterns formed by dividinga drawing object pattern into exposable regions; and drawing the drawingobject pattern by joining portions irradiated with an electron beampassing through the opening patterns of the exposure mask. Each endportion serving as a joint in each opening pattern of the exposure maskis provided with a joining portion tapered in a width of the openingpattern. The exposure is performed in such a way that portions drawnthrough every adjacent joining portions overlap each other.

According to the aspect, a pattern constituting an optical element canbe quickly formed with a smaller number of times of exposure sinceelectron beam irradiation is performed in the shapes obtained bydividing the drawing object pattern, and the time required for theexposure can be shortened while reducing edge roughness.

In addition, each end portion serving as a joint in each opening patternof the exposure mask used for the exposure is provided with a joiningportion tapered in a width of the opening pattern. The exposure isperformed in such a way that portions drawn through every adjacentjoining portions overlap each other. Thus, it is possible to reduce avariation in line width at the joining portions in an exposure regionand to draw the pattern with less roughness.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are perspective views showing a method of manufacturing arace-track optical resonator.

FIG. 2 is a view showing a method (a comparative example) of drawing anoptical resonator by a point beam method.

FIG. 3 is a view showing a problem of the method (the comparativeexample) of drawing an optical resonator using a point beam.

FIG. 4 is a view showing an electron beam exposure apparatus accordingto a first embodiment used for manufacturing an optical resonator.

FIGS. 5A and 5B are views showing operational modes of the electron beamexposure apparatus of FIG. 4.

FIGS. 6A and 6B are views showing a pattern formation method for anoptical resonator according to the first embodiment.

FIGS. 7A to 7C are views showing a method of processing joining portionsof exposure patterns in an electron beam exposure method of the firstembodiment.

FIGS. 8A and 8B are views showing a pattern formation method for anoptical resonator according to a second embodiment.

FIGS. 9A and 9B are views showing a method of processing a joiningportion of a curved pattern and a linear pattern in an electron beamexposure method of FIGS. 8A and 8B.

FIGS. 10A and 10B are views showing a pattern formation method for anoptical resonator according to a third embodiment.

FIGS. 11A to 11D are views showing a relationship between overlaps ofcircular shots and edge roughness.

DESCRIPTION OF EMBODIMENTS

Embodiments will be described below with reference to the accompanyingdrawings.

First Embodiment

A first embodiment will be described citing a race-track opticalresonator as an example of an optical element.

FIGS. 1A to 1C are views showing a method of manufacturing a race-trackoptical resonator, which are arranged in the order of steps.

First, as shown in FIG. 1A, a silicon oxide film 12, a silicon film 13,and a resist film 14 are sequentially formed on a silicon substrate 11.

Next, as shown in FIG. 1B, linear patterns 14 a and an annular pattern14 b are formed by subjecting the resist film 14 to exposure using anelectron beam and to development thereafter.

Subsequently, etching is carried out by using the patterns 14 a, 14 b ofthe resist film 14 collectively as a mask. Thus, portions of the siliconfilm 13 not covered with the resist film 14 are removed, and linearwaveguide patterns 13 a and an annular waveguide pattern 13 b areformed.

Then, the patterns 14 a, 14 b of the resist film 14 are removed as shownin FIG. 1C. A race-track optical resonator 10 including the opticalwaveguides made of silicon is thus finished.

A line width of each of the waveguide patterns 13 a, 13 b of the opticalresonator 10 is set to about 0.5 aam, for example, and each waveguidepattern 13 a is located away from the annular waveguide pattern 13 b bya predetermined distance. When an optical signal is propagated to one ofthe linear waveguide patterns 13 a, an optical signal having apredetermined frequency corresponding to the distance between thewaveguide patterns 13 a, 13 b is propagated and thus extracted to theannular waveguide pattern 13 b.

If there is roughness on edges of the waveguide patterns 13 a, 13 b, theoptical signal leaks out from the rough edges and a signal lossincreases as a consequence. It is therefore necessary to reduce anaverage value of the edge roughness of each of the waveguide patterns 13a, 13 b to less than several nanometers

Meanwhile, since a frequency characteristic of the race-track opticalresonator depends on the gap between each waveguide pattern 13 a and thewaveguide pattern 13 b, gap portions between the patterns need to bemanufactured with high accuracy.

FIG. 2 is a view showing an electron beam exposure method (a comparativeexample) using a point beam.

As shown in FIG. 2, in the electron beam exposure method using the pointbeam, an electron beam 84 emitted from an electron gun 81 passes througha circular aperture of a diaphragm 82, and its cross section is therebyshaped into a circular form. Then, the electron beam 84 is converged toa predetermined size by an electromagnetic lens 83 and delivered as acircular point beam shot 84 a onto a sample. Thereafter, a waveguidepattern is drawn by repeating irradiation with the point beam shot 84 awhile shifting the position of the irradiation with the point beam shot84 a.

The above-described exposure using the point beam is mainly employed inprototype production of an optical element.

FIG. 3 is a view showing a problem of the electron beam exposure method(the comparative example) using the point beam.

As shown in a partially enlarged view B in FIG. 3, the electron beamexposure method using the point beam causes roughness, which occurs onan edge of the waveguide pattern 13 b with a size ΔE corresponding tothe beam size. Accordingly, the beam size has to be reduced in order tolimit the edge roughness of the waveguide pattern 13 b within apredetermined range, and the number of times of the electron beamexposure (the number of shots) has to be increased as a consequence.Hence, there is a problem that the exposure requires a long period oftime.

In the meantime, as shown in a partially enlarged view A, a gap Gbetween each waveguide pattern 13 a and the waveguide pattern 13 b isdefined by an edge of the waveguide. pattern 13 a and an edge of thewaveguide pattern 13 b.

However, it takes time to draw the waveguide pattern 13 b aftercompletion of drawing the edge of the waveguide pattern 13 a, and such atime lag is more likely to cause a displacement in the position ofirradiation with the electron beam or a change in an electric currentvalue of the electron beam.

For this reason, it is difficult to manufacture the gap G between thewaveguide pattern 13 a and the waveguide pattern 13 b precisely in thetargeted size.

This is why this embodiment focuses on a character projection method.

FIG. 4 is a view showing an electron beam exposure apparatus of thisembodiment used for manufacturing an optical resonator.

As shown in FIG. 4, an electron beam exposure apparatus 30 of thisembodiment is broadly divided into an electro-optical system column 100,and a control unit 31 configured to control units of the electro-opticalsystem column 100. Here, the electro-optical system column 100 includesan electron beam generation unit 130, a mask deflection unit 140, and asubstrate deflection unit 150. The pressure inside the electro-opticalsystem column 100 is reduced.

In the beam generation unit 130, an electron beam EB is emitted from anelectron gun 101. The electron beam EB is converged by a firstelectromagnetic lens 102, then passes through a rectangular aperture 103a of a beam shaping mask 103, and its cross section is thereby shapedinto a rectangular form.

Thereafter, the electron beam EB is focused on an exposure mask 110 by asecond electromagnetic lens 105 of the mask deflection unit 140. Firstand second electrostatic deflectors 104, 106 deflect the electron beamEB toward a specific pattern Si formed on the exposure mask 110, wherebythe cross-sectional shape of the electron beam EB is shaped into a formof the pattern Si.

The exposure mask 110 is fixed to a mask stage 123. Here, the mask stage123 is movable within a horizontal plane. In the case of using a patternS located at a position outside a deflection range (a beam deflectionregion) covered by the first and second electrostatic deflectors 104,106, the pattern S is moved into the beam deflection region by movingthe mask stage 123.

Third and fourth electromagnetic lenses 108, 111 located above and belowthe exposure mask 110 plays a role in focusing the electron beam EB on asilicon substrate 11 by adjusting amounts of electric currents to besupplied to those lenses.

The electron beam EB which has passed the exposure mask 110 is returnedto an optical axis C by deflecting actions of third and fourthelectrostatic deflectors 112, 113. Then, the size of the electron beamEB is reduced by a fifth electromagnetic lens 114.

The mask deflection unit 140 is provided with first and secondcorrection coils 107, 109, which are configured to correct deflectionaberrations attributed to the first to fourth electrostatic deflectors104, 106, 112, 113.

Then, the electron beam EB passes through an aperture 115 a of a shieldplate 115 constituting a part of the substrate deflection unit 150, andis projected onto the silicon substrate 11 by first and secondprojection electromagnetic lenses 116, 121. Thus, an image of thepattern on the exposure mask 110 is transferred to the silicon substrate11 at a given reduction ratio such as a reduction ratio of 1/10.

The substrate deflection unit 150 is provided with a fifth electrostaticdeflector 119 and an electromagnetic deflector 120. The electron beam EBis deflected by the deflectors 119, 120, whereby the image of thepattern on the exposure mask is projected to a predetermined position onthe silicon substrate 11.

In addition, the substrate deflection unit 150 is provided with thirdand fourth correction coils 117, 118 configured to correct a deflectionaberration of the electron beam EB on the silicon substrate 11.

The silicon substrate 11 is fixed to a wafer stage 33, which is movablein a horizontal direction, by a drive unit (not shown) such as a motor.It is possible to perform the exposure on the entire surface of thesilicon substrate 11 by moving the wafer stage 33.

Meanwhile, the control unit 200 includes an electron gun controller 202,an electro-optical system controller 203, a mask deflection controller204, a mask stage controller 205, a blanking controller 206, a substratedeflection controller 207, and a wafer stage controller 208. Among them,the electron gun controller 202 controls the electron gun 101, andthereby controls an acceleration voltage of the electron beam EB, a beamemission condition thereof, and the like.

Meanwhile, the electro-optical system controller 203 controls amounts ofelectric currents to be supplied to the electromagnetic lenses 102, 105,108, 111, 114, 116, and 121 as well as other parameters and therebyadjusts magnifying power, a focal position, and the like of theelectro-optical system formed from these electromagnetic lenses. Theblanking controller 206 controls an application voltage to a blankingelectrode 127. Thus, the blanking controller 206 deflects the electronbeam EB having been generated before starting the exposure onto theshield plate 115 and thereby prevents the electron beam EB from beingdelivered onto the silicon substrate 11 prior to the exposure.

The substrate deflection controller 207 controls an application voltageto the fifth electrostatic deflector 119 and an amount of electriccurrent to be supplied to the electrostatic deflector 120, therebydeflecting the electron beam EB toward a predetermined position on thesilicon substrate 11.

The wafer stage controller 208 moves the silicon substrate 11 in thehorizontal direction by adjusting a drive amount of a drive unit 125,thereby delivering the electron beam EB to a desired position on thesilicon substrate 11. The controllers 201 to 208 are operated on thebasis of exposure data which prescribes exposure conditions for eachexposure shot supplied from an integrated control system 34. Theexposure data are created by the integrated control system 34 formedfrom a workstation, for example.

The above-described electron beam exposure apparatus 30 can performexposure in accordance with a variable shaped beam (VSB) method and acharacter projection (CP) method. FIG. 5A is a view explaining thevariable shaped beam method and FIG. 5B is a view explaining thecharacter projection method.

As shown in FIG. 5A, in the variable shaped beam (VSB) method, part ofthe electron beam EB having passed through the rectangular aperture 103a of the beam shaping mask 103 is cut out by using a rectangular openingpattern S0 provided in the exposure mask 110. Then, the rectangularelectron beam EB having passed through the rectangular opening patternS0 is delivered onto the silicon substrate 11.

The above-described VSB method can produce electron beam shots invarious rectangular shapes by adjusting the degree of overlap betweenthe rectangular opening pattern S_(o). and the electron beam havingpassed through the rectangular aperture 103 a. Moreover, unlike in thepoint beam, an electric current density of the electron beam does notvary with a change in size of the electron beam shot. Accordingly,exposure time does not change even in the case of performing theexposure in a large area in a lump. As a consequence, the exposure inthe large area in a lump makes it possible to achieve a drawing speedwhich is several tens to several hundreds times faster than the pointbeam method.

On the other hand, as shown in FIG. 5B, in the case of the CP method,the electron beam EB having passed through the rectangular aperture 103a of the beam shaping mask 103 is caused to further pass through any oneof opening patterns S1 to S4 provided in the exposure mask 110. Thus, anelectron beam shot is delivered in the same cross-sectional shape as theopening pattern which the electron beam has passed through.

In the CP method, an edge of the delivered electron beam EB is cut outby using the opening pattern Si of the exposure mask 110. Accordingly,any blurs of the electron beam EB generated by the electro-opticalsystem located above the exposure mask 110 are not propagated. For thisreason, the CP method can obtain sharper edges of the electron beam EBand higher resolution than the VSB method. In addition, the CP methodcan draw a complicated shape other than a rectangle in a single exposureoperation. Accordingly, by preparing appropriate opening patterns, theCP method can also draw a pattern including a curved portion byperforming a smaller number of times of exposure operations than thoserequired in the case of using only the VSB method.

Here, the rectangular opening pattern S0 shown in FIG. 5A and theopening patterns S1 to S4 shown in FIG. 5B may be formed on the sameexposure mask 110.

In this embodiment, the exposure of patterns is performed by using onlythe CP method out of the above-described exposure methods.

FIG. 6A shows an exposure mask used in exposure for a race-track opticalresonator according to the first embodiment and FIG. 6B is a viewshowing an exposure method for the race-track optical resonator usingthe exposure mask of FIG. 6A.

As shown in FIG. 6B, in this embodiment, optical waveguides constitutingthe race-track optical resonator are divided into a plurality of smallregions 211 a to 211 n.

Here, the gap portions between the linear waveguide patterns 13 a andthe annular waveguide pattern 13 b constituting the race-track opticalresonator are each defined as a portion including parallel linear lines,and each portion including the parallel linear lines is defined as asingle small area 211 b.

Meanwhile, each linear waveguide pattern 13 a is divided into aplurality of small regions 211 a whereas the annular pattern 13 b isdivided into small regions 211 c to 211 n.

Moreover, as shown in FIG. 6A, opening patterns 212 a to 212 ncorresponding to the respective divided regions of the waveguidepatterns are formed on an exposure mask 210.

Hence, the waveguide patterns 13 a, 13 b are drawn by performing theexposure of the divided regions in accordance with the CP method usingthe exposure mask 210.

Next, a method of processing joining portions of divided patterns willbe described.

FIG. 7A is an enlarged view of an end portion of the opening pattern,FIG. 7B is a view showing an exposure method for joining portions of thewaveguide patterns, and FIG. 7C is a view showing the joining portionsof the waveguide patterns according to the method shown in FIG. 7B.

As shown in a partially enlarged view of FIG. 7A, an end portion of theopening pattern 212 a of this embodiment includes a joining portion 213a, which is not simply formed from a right-angle edge only, but isinstead formed from a slightly curved edge.

In this, embodiment, the exposure is performed by locating exposurepositions of two adjacent opening patterns 212 a in such a way that thejoining portions 213 a thereof partially overlap each other as shown inFIG. 7B. In FIG. 7B, wedge-shaped recesses 214 a are formed at a jointof the patterns. However, the recesses 214 a are compensated by thethickening of pattern widths by exposing the overlapping joiningportions 213 a twice.

As a result, the waveguide pattern 13 a without any recesses at thejoint can be manufactured as shown in FIG. 7C.

According to the pattern exposure method for a race-track opticalresonator of this embodiment, the electron beam exposure is performed inaccordance with the CP method after dividing the waveguide patterns intothe predetermined number of regions. Thus, the race-track opticalresonator 10 can be formed by performing a small number of times of theexposure operations while reducing the roughness of the pattern edges.As a consequence, it is possible to significantly reduce the timerequired for the exposure as compared to the case of using the pointbeam method.

In addition, each gap portion 211 b which is to determine thecharacteristic of the optical resonator 10 is formed by one exposureoperation using a gap opening pattern 212 b (FIG. 6A) configured by twoparallel openings with a gap therebetween. Thus, it is possible to formthe gap portion with extremely high accuracy while avoiding effects ofdrift of the electron beam and a variation in the electric currentvalue.

Second Embodiment

FIGS. 8A and 8B are views showing an electron beam exposure methodaccording to a second embodiment.

As shown in FIG. 8A, circular opening patterns 222 c, 222 d, and 222 eare formed in an exposure mask 220 of this embodiment as well as alinear opening pattern 222 a and a gap opening pattern 222 b. Inaddition, guard regions 222 with no opening patterns are provided inpredetermined ranges above and below as well as on the right and left ofeach of the circular opening patterns 222 c, 222 d, and 222 e.Illustration of some of the guard regions corresponding to the openingpatterns 222 d, 222 e is omitted in FIG. 8A.

As noted above, the exposure mask 220 is also provided with a linearopening pattern 222 a for forming a linear waveguide pattern, and a gapopening pattern 222 b for forming a gap portion. As shown, the gapopening pattern 222 b is configured by two parallel openings betweenwhich a predetermined gap is formed.

As shown in FIG. 8B, the linear waveguide patterns 13 a and the gapportions between the waveguide patterns 13 a and the waveguide pattern13 b are drawn by the exposure in accordance with the CP method usingthe opening patterns 222 a, 222 b in this embodiment as well.

On the other hand, a curved portion of the annular waveguide pattern 13b is drawn by the exposure while shifting and overlapping electron beamshots 224 shaped with the opening pattern 222 c. Each circular electronbeam shot 224 has a diameter which is substantially equal to the linewidth of the waveguide pattern 13 b.

In this case, the exposure using the circular electron beam shots 224overlaps the exposure through the opening pattern 222 at joints betweenthe curved portions and the gap portions of the waveguide pattern 13 b,whereby the line width is more likely to be thickened at the joints.

To avoid such thickening, joining portions of the curved portions andthe gap portions of the waveguide pattern 13 b are subjected to theexposure in accordance with the following method in this embodiment.

FIGS. 9A and 9B are views showing a method of processing a joint betweena curved portion and a gap portion in the electron beam exposure methodof this embodiment.

In this embodiment, as shown in FIG. 9B, the exposure is performed insuch a way that the circular electron beam shot does not extend off froma terminal edge portion of the curved portion of the waveguide pattern13 b. This makes it possible to avoid the overlap of the exposurebetween the gap portion and the curved portion and to avoid thickeningof the line width at the joint of the gap portion and the curvedportion.

As shown in FIG. 9A, the above-described prevention of the circularelectron beam shot from extending off is achieved by adjusting anoverlap between the opening pattern 222 c of the exposure mask 220 and arectangular beam shot 225 shaped by the rectangular aperture 103 a ofthe beam shaping mask 103. Specifically, when the position ofirradiation with the circular electron beam shot 224 comes close to aboundary between the curved portion and the gap portion, the position ofirradiation with the rectangular electron beam shot 225 is shifted.Hence, the position of irradiation is adjusted in such a way that onlypart of the rectangular beam shot 225 overlaps the opening pattern 222c. Thus, the circular electron beam shot 224 can be prevented fromextending off into the gap portion side.

In this case, the guard regions 222 without any formed patterns areprovided above and below as well as on the right and left of the openingpattern 222 c of the exposure mask 220. For this reason, when theposition of irradiation of the rectangular electron beam shot 225 isshifted for processing the joint of the curved portion, the electronbeam can be prevented from passing through any opening patterns otherthan the opening pattern 222 c.

As described above, in this embodiment, the curved waveguide pattern isformed by joining the electron beam shots having the circular crosssection. This makes it possible to form waveguide patterns havingvarious curvatures.

In addition, it is possible to reduce the number of times of exposureoperations as compared to the point beam method. Thus, the time requiredfor drawing the waveguide patterns is shortened.

Third Embodiment

FIGS. 10A and 10B are views showing an electron beam exposure methodaccording to a third embodiment.

As shown in FIG. 10A, an exposure mask 230 of this embodiment isprovided with a linear opening pattern 232 a for forming the linearwaveguide pattern 13 a, a gap opening pattern 232 b for forming the gapportion, and a circular opening pattern 232 c. Note that, although notshown, the exposure mask 230 of this embodiment is further provided witha rectangular opening pattern such as an opening S_(o). shown in FIG. 5Aso as to perform electron beam exposure including a variable shaped beam(VSB) method.

A diameter of the circular opening pattern 232 c is formed smaller insize than line widths of the waveguide patterns 13 a, 13 b.

As shown in FIG. 10 b, the linear waveguide patterns 13 a and the gapportions between the waveguide patterns 13 a and the waveguide pattern13 b are subjected to exposure in accordance with the CP method, as inthe case of the method shown in FIG. 6.

As shown in a partially enlarged view in FIG. 10B, regarding the curvedportion of the waveguide pattern 13 b, a portion near each edge wherereduction in roughness is required is subjected to the exposure by usingcircular electron beam shots 231 c.

Meanwhile, as shown in FIG. 10B, an internal part of the curved portionwhich requires relatively low accuracy is subjected to exposure inaccordance with the VSB method described above with reference to FIG.5A. Since the exposure in the VSB method is used as well, it is possibleto draw the pattern by performing a small number of times of exposureoperations as compared to the case of using only the point beam.

Here, a pitch of the circular electron beam shots 231 c is determined bythe following method in compliance with the diameter of the electronbeam shots and the required roughness.

FIGS. 11A to 11D are views showing a relationship between the pitch ofthe circular electron beam shot 231 c and the size of the edgeroughness.

FIG. 11A shows a case where a pitch P1 of the circular electron beamshots 231 c is set equal to a diameter W of each circular electron beamshot 231 c. In this case, roughness δ₁ which is half as large as thediameter W of the electron beam shot occurs as shown in FIG. 11A.

FIG. 11B shows a case where a pitch P2 of the electron beam shots 231 cis set half as large as the diameter W of the circular electron beamshot 231 c. In this case, roughness δ₂ which is about 0.067 times aslarge as the diameter W of the electron beam shot occurs as shown inFIG. 11B.

For example, when the diameter W of the electron beam shot 231 c is setto 60 nm and the pitch of the electron beam shots 231 c is set half aslarge as the diameter W, the roughness occurring in this case isestimated to be about 4.02 nm. This value has been confirmed to coincidewith an experiment result obtained by the inventors of this application.

FIG. 11C shows a case where a pitch P3 of the electron beam shots 231 cis set a quarter as large as the diameter W of the circular electronbeam shot 231 c. Roughness δ₃ which is about 0.032 times as large as thediameter W of the electron beam shot occurs in this case.

Furthermore, FIG. 11D shows a case where a pitch P4 of the electron beamshots 231 c is set one-eighth as large as the diameter W of the circularelectron beam shot 231 c. Roughness δ₄ which is about 0.008 times aslarge as the diameter W of the electron beam shot occurs in this case.

As described above, the roughness occurring at the edge portions becomesless as the pitch of the circular electron beam shots 231 c is setsmaller.

To reduce the number of times of the exposure operations, it isdesirable to set the pitch of the circular electron beam shots 231 c aslarge as possible insofar as the tolerable edge roughness can beachieved.

When the electron beam 231 c shots having the diameter of 60 nm areused, for example, the roughness can be reduced to about 4 nm by settingthe pitch to 30 nm (½ W). Thus, it is possible to obtain a sufficientoutcome as the waveguide pattern.

In this embodiment, the pitch of the circular electron beam shots 231 cat a peripheral edge on an inner peripheral side of the curved portionof the waveguide pattern 13 b in FIG. 10B is set equal to the pitch ofthe circular electron beam shots 231 c at a peripheral edge on an outerperipheral side thereof. Thus, it is possible to prevent a problem of adifference in size of roughness between the inner peripheral side andthe outer peripheral side of the waveguide pattern 13 b.

In this embodiment, it is possible to draw curved patterns in varioussizes without being limited by the diameter of the circular electronbeam shot 231 c. In addition, it is possible to draw a large-areapattern by a small number of times of the exposure operations by usingthe exposure in the VSB method as appropriate, and thereby tomanufacture the optical element promptly.

What is claimed is:
 1. An electron beam exposure method comprising thesteps of: preparing an exposure mask including a rectangular openingpattern, a linear opening pattern, a gap opening pattern, and a circularopening pattern having a diameter smaller than a width of the linearopening pattern; drawing each edge of a drawing object pattern byrepeating exposure with an electron beam passing through the circularopening pattern while shifting and overlapping exposure positions of theelectron beam; and drawing a portion inside the edges of the drawingobject pattern by repeating the exposure with an electron beam passingthrough the rectangular opening pattern while shifting and overlappingexposure positions of the electron beam.
 2. The electron beam exposuremethod according to claim 1, wherein a pitch of positions of theexposure with the electron beam passing through the circular openingpattern is set to a constant distance along the edge of the drawingobject pattern.
 3. The electron beam exposure method according to claim2, wherein a pitch of the positions of irradiation with the electronbeam passing through the circular opening pattern is set to a half of adiameter of the electron beam passing through the circular openingpattern, and the diameter of the electron beam passing through thecircular opening pattern is set equal to or below 1/0.067 times a size δof tolerable edge roughness.
 4. The electron beam exposure methodaccording to claim 1 further comprising a step of drawing a linearportion of the drawing object pattern by repeating exposure with theelectron beam passing through the linear opening pattern.
 5. Theelectron beam exposure method according to claim 1 further comprising astep of drawing a gap portion of the drawing object pattern by oneexposure operation with an electron beam passing through the gap openingpattern.
 6. The electron beam exposure method according to claim 5wherein the gap opening pattern is configured by two parallel openingswith a gap therebetween.